This invention relates generally to fuel cell systems, and more particularly to methods and systems for humidifying a supply of gas, e.g., fuel and air, for use in fuel processors and fuel cell systems.
Fuel cells electrochemically convert reactants, for example, a fuel and an oxidant, to electricity. Unlike batteries, which typically contain a set amount of chemicals for generating electricity and which stop delivering electricity once the chemicals are consumed, fuel cells can deliver electricity continuously as long as the fuel cells receive a fuel and an oxidant.
A Proton Exchange Membrane (hereinafter xe2x80x9cPEMxe2x80x9d) fuel cell converts the chemical energy of reactants such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H+ ions) from the xe2x80x9canodexe2x80x9d side of a fuel cell to the xe2x80x9ccathodexe2x80x9d side of the fuel cell while preventing passage therethrough of the reactants (e.g., hydrogen and air/oxygen).
In PEM fuel cells, typically the membrane works more effectively if it is wet. Conversely, once any area of the membrane dries out, the electrochemical reaction in that area stops. Eventually, the dryness can progressively march across the membrane until the fuel cell fails completely. As a result, the fuel and oxidant fed to each fuel cell are usually humidified, e.g., with steam.
Where pure reactants are not readily available or economical to supply to a fuel cell, it may be desirable to use air as an oxygen source, and to use a fuel processor to convert a hydrocarbon such as methane or methanol into a hydrogen-rich stream. The two reactions which are generally used to achieve this conversion as shown in equations (1) and (2).
xc2xdO2+CH4xe2x86x922H2+COxe2x80x83xe2x80x83(1) 
H2O+CH4xe2x86x923H2+COxe2x80x83xe2x80x83(2) 
The reaction shown in equation (1) is sometimes referred to as catalytic partial oxidation (CPO). The reaction shown in equation (2) is generally referred to as steam reforming. A fuel processor may use either of these reactions separately, or both in combination. While the CPO reaction is exothermic, the steam reforming reaction is endothermic. A reactor utilizing both reactions to maintain a relative heat balance is sometimes referred to as an autothermal reactor (ATR). Also, it should be noted that fuel processors are sometimes generically referred to as reformers, and the fuel processor output gas is sometimes generically referred to as reformate, without respect to which reaction is employed.
As evident from equations (1) and (2), both reactions produce carbon monoxide (CO). Because of the high temperature at which the fuel processor is operated, this CO generally does not affect the catalysts in the fuel processor. However, if this reformate is passed to a fuel cell system operating at a lower temperature (for example, less than 100 degrees C.), the CO may poison the catalysts in the fuel cell by binding to catalyst sites, inhibiting the hydrogen in the cell from reacting. In such systems it is typically necessary to reduce CO levels to less than 100 parts per million (ppm). For this reason the fuel processor may employ additional reactions and processes to reduce the CO that is produced. For example, two additional reactions that may be used to accomplish this objective are shown in equations (3) and (4). The reaction shown in equation (3) is generally referred to as the shift reaction, and the reaction shown in equation (4) is generally referred to as preferential oxidation (PROX).
CO+H2Oxe2x86x92H2+CO2xe2x80x83xe2x80x83(3) 
CO+xc2xdO2xe2x86x92CO2xe2x80x83xe2x80x83(4) 
As evident from equations (2) and (3), water may be employed as a reactant in a fuel processing system. It thus may be desirable to control the amount of water added to the fuel processor, for example to control the temperature of an ATR or reforming reactor, or to drive the shift reaction to eliminate carbon monoxide. In the case of ATRs and reforming reactors, the amount of water in feed streams to such reactors is generally referred to as the steam-to-carbon ratio.
For example, a fuel may be humidified with steam prior to entering the fuel processor. Another approach provides a supply of fuel and air at ambient temperature which is humidified with heated water prior to entering the fuel processor.
There is a need for improvements in methods and systems for humidifying fuel for use in a fuel processor and improvements in fuel cell system efficiency.
The present invention provides, in a first aspect, a method for humidifying a supply of fuel and air for use in a fuel processor in which the method includes receiving the supply of fuel and air at a first temperature, heating the supply of fuel and air to a second temperature greater than the first temperature, and combining the supply of fuel and air at the second temperature with a supply of water.
The present invention provides, in a second aspect, a method for generating electricity in which the method includes receiving a supply of fuel and air at a first temperature, heating the supply of fuel and air to a second temperature greater than the first temperature, combining the supply of fuel and air at the second temperature with a supply of water, reforming the combined supply of fuel, air, and water, and reacting the reformed supply of fuel, air, and water with an oxidant to generate electricity.
The present invention provides, in a third aspect, fuel processor for reforming a supply of fuel and air for a fuel cell in which the fuel processor includes a first heat exchanger for heating a supply of fuel and air at a first temperature to a second temperature greater than the first temperature prior to humidification of the supply of fuel and air, an autothermal reactor for receiving the supply of fuel and air after humidification, at least one of a high temperature shift device and a low temperature shift device operably connected to the autothermal reactor, and a preferential oxidation device, operably connected to the at least one of the high temperature shift device and the low temperature shift device, for discharging a supply of reformate.
The present invention provides, in a fourth aspect, a fuel cell system which includes means for heating a supply of fuel and air at a first temperature to a second temperature greater than the first temperature, means for humidifying the supply of fuel and air at the second temperature, means for reforming the supply of humidified fuel and air, and means for reacting the reformed supply of humidified fuel and air with an oxidant to generate electricity.
The present invention provides, in a fifth aspect, a fuel cell system which includes a first heat exchanger for heating a supply of fuel and air to a second temperature greater than the first temperature, a humidifier for humidifying the supply of fuel and air at the second temperature, a fuel processor for reforming the supply of humidified fuel and air, and a fuel cell for reacting the reformed supply of humidified fuel and air with an oxidant to generate electricity.